1. Field of the Invention
The invention provides a control circuitry for controlling the rotational speed of a DC motor, and more particularly to a control circuitry that modulates the rotational speed by switching the operation mode of a DC motor.
2. Related Art
The processing and spreading of vast amounts of electronic data has facilitated the rapid exchange of information and knowledge, accelerated technological development and enriched our lives. However, when processing large numbers of data transfers, the central processing unit (CPU) of a laptop computer, for example, is prone to overheating. Therefore, it is essential that devices like laptop computers have an excellent heat dissipation device with minimal power consumption to eliminate the problem of overheating.
Referring to FIG. 1, it is a block diagram of a heat-dissipating process of a CPU 12 as performed by a conventional heat dissipation device 10. As shown in FIG. 1, the heat dissipation device 10 includes a DC motor 14, a driving circuit 16 and a fan 18. The driving circuit 16 and the fan 18 are both electrically connected to the DC motor 14. When the heat dissipation device 10 dissipates heat from the CPU 12, the driving circuit 16 firstly transmits a rotation signal, usually a current signal, in order to control the rotation of the DC motor 14. Next, the fan 18 rotates in accordance with the rotation of the DC motor 14 to cool the CPU 12.
Generally, the fan 18 is directly disposed on the DC motor 14, so that the rotational speed of the fan 18 is same as the rotational speed of the DC motor 14. When heat generated by the CPU 12 increases, the current signal outputted from the driving circuit 16 increases gradually, and the rotational speed of the DC motor 14 and the fan 18 increases as well. When the heat generated by the CPU 12 is relatively small, such as when little data is being processed, the DC motor 14 is not required to turn as fast. So, the driving circuit 16 outputs a less current to the DC motor 14 for powersaving.
Normally, the characteristics of a DC motor are set after the design stage, and a direct proportion is between the input voltage and the rotational speed of the DC motor. Referring to FIG. 2, it is a graph of input voltage versus rotational speed of the DC motor 14. Suppose that the relationship between the input voltage and rotational speed of the DC motor 14 is represented by a characteristic curve T1 So, when the input voltage to the DC motor 14 is 5 volts, the rotational speed of the DC motor 14 is 4000 rpm, and when the input voltage to the DC motor 14 is 2.5 volts, the rotational speed of the DC motor 14 is 2000 rpm. However, the rotational speed of the DC motor 14 is relatively too high at low input voltage (i.e. the input current is relatively too high).
The coil windings inside the DC motor 14 can be redesigned in order to reduce the motor's rotational speed at low input voltages. For example, the DC motor 14 can be designed to operate at 1500 rpm for an input voltage of 2.5 volts and the relationship between the input voltage and rotational speed of the redesigned DC motor 14 follows a new characteristic curve T2. Although the goal of dropping the rotational speed at lower input voltages has been achieved by following the characteristic curve T2, the rotational speed at higher voltages has been greatly compromised. As shown in FIG. 2, the rotational speed at an input voltage of 5 volts is only 3500 rpm.